In recent years, a high-speed and high-capacity optical communication system has been realized. To incorporate into such a high-speed and high-capacity optical communication system, the development of a small, high speed, low cost and highly stable optical modulator has been demanded.
As an optical modulator to meet such demand, there is a lithium niobate optical modulator of traveling-wave electrode type (hereinafter referred to as an LN optical modulator) in which an optical waveguide and a traveling-wave electrode are formed on a substrate, such as lithium niobate (LiNbO3) (hereinafter referred to as an LN substrate), which has a so-called electro-optical effect, i.e., an effect of refraction index being changed by an electrical field applied. This LN optical modulator has been applied to high-capacity optical communication systems of 2.5 Gbit/s and 10 Gbit/s because of its excellent chirping characteristics. Recently, an application to an ultrahigh-capacity optical communication system of 40 Gbit/s is being further considered.
Now, LN optical modulators that have been put into practical use or been proposed utilizing the electro-optical effect of lithium niobate will be described below.